This invention relates in general to a process for fabricating electrophotographic imaging members, and, more specifically, to the formation of a charge transport layer
Typical electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated, for example, in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. The disclosure of this patent is incorporated herein in its entirety. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer the photogenerating layer is sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge. The photosensitive member is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in illuminated areas of the photosensitive member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electrostatic toner particles on the surface of the photosensitive member. The resulting visible toner image can be transferred to a suitable receiving material such as paper. This imaging process may be repeated many times with reusable photosensitive members.
As more advanced, complex, highly sophisticated, electrophotographic copiers, duplicators and printers were developed, greater demands were placed on the photoreceptor to meet stringent requirements for the production of high quality images. For example, the numerous layers found in many modern photoconductive imaging members must be uniform, free of defects, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a drum or belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an overcoating layer. Although excellent toner images may be obtained with multilayered photoreceptors, it has been found that the numerous layers limit the versatility of the multilayered photoreceptor. For example, when a thick, e.g., 29 micrometers, layer of a charge transport layer is formed in a single pass a raindrop pattern to form on the exposed imaging surface of the final dried photoreceptor. This raindrop phenomenon is a print defect caused by the coating thickness variations (high frequency) in photoreceptors having a relatively thick (e.g., 29 micrometers) charge transport layer. More specifically, the expression "raindrop", as employed herein, is defined as a high frequency variation in the transport layer thickness. The period of variation is in the 0.1 cm to 2.5 cm range. The amplitude of variation is between 0.5 micrometer and 1.5 micrometers. The variation can also be defined on a per unit area basis. Raindrop can occur with the transport layer thickness variation is in the range of 0.5 to 1.5 microns per sq. cm. The morphological structure of raindrop is variable depends on where and how the device is coated. The structure can be periodic or random, symmetrical or oriented.